No shit, I came here to point this out. Like, wtf does the WESTERN Roman Empire part really have to do with anything? I want to see what kinds of Mechas the Byzantines and even various Caliphates could have eventually churned-out if their reigns hadn't ended as much as I begin to give a damn what various stages of Rome, specifically, would have done.
Same goes for Egypt, Ghenghis Khan and all of the African empires that time(and disinterest) has ground to presently un-known dust and rumor. Even random Greek city-states' offerings would at least be unique and not guided by OOP's/our misunderstandings about Rome's "super-marbled"/black-and-white-and-gold-and-red aesthetics.
Fuckers had "machine guns" in their day ... the miracle is that anyone else got a chance to show us something cool in the times since.